The five commonly observed power supply line disturbances are voltage surges, voltage sags, overvoltage, under-voltage, and brownouts. Of these, voltage sags account for 90-95% of events, particularly in commercial and industrial facilities. Voltage sags are voltage reductions in the nominal line voltage. Typically, voltage sags are characterized by drops of between 10%-90% of nominal (system) line voltages. The drops in voltage typically last from a cycle (16.6 millisecond) to a second or so, or tens of milliseconds to hundreds of milliseconds.
The reason for occurrence of voltage sags can be due to faults on the grid, and also due to high starting currents drawn by electrical loads (e.g., motors, refrigerators, freezers, air conditioners, etc.) at startup. Another reason for occurrence of voltage sags is faults in the power provider's transmission or distribution lines. Voltage sags occurring at high voltages typically spread through the electrical utility network and are transmitted to lower voltage systems via line transformers. Additionally, voltage sags can occur frequently in some locations that experience severe weather phenomenon such as lightning, wind, and ice. For example, lightning strikes a power line and continues to ground, which results in a line-to-ground fault. The line-to-ground fault in turn creates a voltage sag and this reduced voltage can be seen over a wide area. The amplitude of a voltage sag is the value of the instantaneous line voltage during a voltage sag. Generally, voltage sags are followed by a short duration increase (i.e. inrush) in the line current upon to nominal voltage levels due to discharge of reactive impedance in the load during the sag.
Power protection equipment to date has focused primarily on protecting downstream (i.e. from the perspective of the power supply) electrical equipment from damage. Further, since typical power protectors are simple inexpensive electrical devices, having little or no power electronics for fast dynamic control, these devices have not been used to provide ride-through during an electrical line disturbance. Examples of typical power protectors include (but are not limited to) Metal Oxide Varistors (MOVs), relays, thermistors such as Negative Temperature Coefficient (NTC) thermistors or Positive Temperature Coefficient (PTC) thermistors, etc.
For example, MOVs are used for protection against lightning strikes. NTC thermistors or PTC thermistors are inserted for protection against inrush current, and relays are used to cut out equipment in case of damaging overvoltage events. However, neither relays nor thermistors are able to provide ride through functionality. Ride through functionality involves providing temporary electrical energy to synthesize normal operating conditions for a connected load or electrical equipment, during the occurrence of momentary electrical disturbances such as voltage sags. Such a functionality can be provided by a single device, or a combination of electrical components connected in a certain arrangement. A ride-through device that is in common use is an uninterruptible power supply (UPS), or a voltage sag corrector, such as the dynamic sag corrector. However, these devices generally do not provide any protection functionality.
Therefore, it can be appreciated that what is needed is a device that protects against common disturbances and also allows the machine/load/equipment to keep operating through frequently occurring disturbances (e.g., voltage sags). Voltage sags are voltage reductions in the line voltage.
From various electrical power grid measurements, it has been determined that voltage sags are statistically distributed in a manner such that a vast majority of sags retain at least 50% of the nominal line voltage value and last no more than 2-3 seconds. Conventionally, a typical sag correction device would involve the use of an inverter to inject the additional voltage needed, during the occurrence of a voltage sag, with a normal bypass arrangement to restore back to normal operating line conditions when the sag is over. However, such sag correction devices are typically very expensive.
Another possibility is to use an ac chopper arrangement to boost the incoming line voltage to an appropriate value. Such a boost converter arrangement is well known to practicing engineers. However, boost converters suffer from a significant limitation in terms of speed of response—driven by a ‘right half plane’ zero in the control characteristics. Further, the need for gate drives and control logic to interconnect each element of the ac switches, adds to cost and ‘real estate’ in a product that is preferred to be compact and low-cost.
What is proposed in this disclosure is a low-cost arrangement of transient voltage surge protection devices such as MOVs, relays for sustained overvoltage and under-voltage protection, and an arrangement of semiconductor devices such as MOSFETs and diodes—along with their control, power supply and gate drive circuits, that provide protection against the common disturbances, as well as ride-through for connected equipment, in the presence of frequently occurring power disturbances—i.e. voltage sags. Thus, there is clearly a need for a combined inexpensive device that integrates protection and ride-through functions and does so without sacrificing the cost of the typical power protector.